Whitney Isenhower looks at how researchers are turning to heat-tolerant corals to help save declining coral reefs amid the climate crisis.

Austin Bowden-Kerby, a pioneer in coral reef conservation, spends many of his days gardening corals for reefs around Fiji and the Pacific. He grows corals in ocean nurseries. Once they’re healthy enough, he moves them to outer ocean areas with the hope they will replicate and grow.

“We’re looking at what Mother Nature would do on her own if she had 1,000 years to adapt,” said Bowden-Kerby, who founded the UNESCO-endorsed Reefs of Hope strategy. “We would have these kinds of things happening.”

Bowden-Kerby is one of several scientists trying to conserve, replicate and reproduce heat-resistant corals before climate change wipes them out.

The United States National Oceanic and Atmospheric Administration has said the world is experiencing a fourth global coral bleaching event. They’ve found that bleaching-level heat stress affected almost 85pc of the world’s coral reef area between 2023 and 2025.

Bleaching causes corals to lose their food source and, with it, their colour. Most corals survive in temperatures between 20 and 29C. But as ocean temperatures rise, it’s difficult for many to thrive.

But naturally occurring, heat-resistant corals can survive in waters up to 36C and potentially higher. They are usually found in warmer waters, like parts of the Pacific Ocean and the Persian Gulf. These corals are increasingly important as sea temperatures rise. So scientists are turning to them to help save declining reefs.

Heat-resistant corals

Corals reefs are extremely diverse places, with around 6,000 coral species worldwide. Reefs are home to more than 4,000 species and 25pc of global marine life. When healthy, corals nurture fish that feed communities, protect shores from floods and storms and boost economies through tourism.

However, heatwaves have led to widespread coral bleaching and loss. When waters become too warm, corals expel the algae in their tissues that give them their colour. That causes corals to turn completely white.

Coral reefs and their ecosystems are also threatened by pollution, ocean acidification, coastal development and overfishing.

Christopher Cornwall, a lecturer in marine biology at Te Herenga Waka-Victoria University of Wellington in New Zealand, co-authored a recent review that found some reefs can survive if corals become more heat-tolerant.

He told me there are multiple things to consider when conserving and replicating corals: restoring heat-resistant corals where it’s feasible, doing so at a large enough scale and maintaining coral diversity. Restored corals also must be able to survive, he added.

“We can’t just do coral restoration without thermally tolerant corals, because they’re just going to die the next time it gets too hot,” Cornwall said.

Assisted evolution

“A lot of the research now is about, can you scale up restoration and how do you do it more effectively?” said Peter Mumby, a professor of coral reef ecology at the University of Queensland in Australia. “One of the key concerns is to make sure those corals are as tolerant of high temperature as possible.”

Breeding heat-tolerant corals is a form of assisted evolution. Humans intervene to speed up natural processes to help corals more quickly respond to and recover from their stressors, like heatwaves from climate change.

One recent study examining the possible success of assisted evolution interventions like breeding and selecting traits found these interventions can help corals become more tolerant to heatwaves, but they need “extremely strong selection”.

Liam Lachs co-authored that study. Lachs is a former postdoctoral research associate in the CoralAssist lab, a team of scientists led by James Guest at Newcastle University in the United Kingdom. Lachs specialises in coral reef ecosystems and researches coral in Palau, a Pacific island country where corals are surviving in warmer waters.

He told me variability within and among reefs and coral species must be considered when creating more heat-resistant coral, which makes replication complex. “Even within a single reef, there’s a range of tolerance levels,” he said.

Algae and bacteria

Researchers at the Australian Institute of Marine Science (AIMS) have found that some algae (Durusdinium), which symbiotically live in corals and provide them with food in exchange for housing and protection, can boost corals’ heat tolerance.

Madeleine van Oppen is a senior principal research scientist at AIMS. She co-authored a recent review about potentially introducing beneficial bacteria into corals to improve their heat tolerance.

Scientists are also exploring whether heat-tolerant corals should be planted across oceans – from the Indo-Pacific region to the Caribbean – and not just in nearby waters.

Van Oppen said new ventures ultimately need more research, and the real test of success is if something done in a lab works in the wild. “Field testing, I’d say, is the next big thing,” she said. “Finding out whether these interventions can enhance tolerance at ecologically relevant scales. Is it stable over time?”

AIMS researchers also found that heat tolerance could be passed down by interbreeding wild colonies of the same coral species. Heat-resistant coral species include some pocillopora and acropora.

If left unchecked, the sustained global temperature is on target to rise more than 1.5C. Some evidence has shown that 70 to 90pc of tropical coral reefs could go extinct even if global warming is limited to 1.5C.

Prior to the fourth event, the Earth already experienced three mass coral bleaching events over the last few decades. An El Niño is expected this year, bringing with it hotter sea surface temperatures, much like in 2024.

For all the efforts by scientists to save coral reefs and ensure heat resilience, nothing will keep corals healthy more than lowering the global temperature. “The lower we can get our greenhouse gas emissions, the more chance there will be that reefs will exist in the future,” said Cornwall.

By Whitney Isenhower

Whitney Isenhower is a freelance journalist and communications consultant specializing in health, climate and science. Her work has appeared in Devex, Live Science and North Carolina Health News. Previously, she supported U.S. Agency for International Development projects until 2025. She also completed the fellowship in journalism and health impact through the University of Toronto Dalla Lana School of Public Health.

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